Fibers or fibrous materials are often used as reinforcements in composite materials. Glass and other ceramic fibers are commonly manufactured by supplying the ceramic in molten form to a bushing, drawing fibers from the bushing, applying a chemical treatment, such as a size, to the drawn ceramic fibers and then gathering the sized fibers into a tow or strand. There are basically three known general types of chemical treatments--solvent-based systems, melt-based systems and radiation cure-based systems.
In a broad sense, the solvent-based chemical treatments include organic materials that are in aqueous solutions (i.e., dissolved, suspended, or otherwise dispersed in water), as well as those that are dissolved in organic solvents. U.S. Pat. Nos. 5,055,119, 5,034,276 and 3,473,950 disclose examples of such chemical treatments. The solvent (i.e., water, organic solvent, or other suitable solvent) is used to lower the viscosity of the chemical treatment to facilitate wetting of the glass fibers. The solvent is substantially unreactable with the other constituents of the chemical treatment and is driven out of the chemical treatment after the wetting of the glass fibers. In each process for applying solvent-based chemical treatments, an external source of heat or some other device external to the fibers is used to evaporate or otherwise remove the water or other solvent from the applied chemical treatment, leaving a coating of organic material on the glass fibers. One drawback to a solvent-based process is that the added step of removing the solvent increases production costs. In addition, some organic solvents are very flammable in vapor form and pose a fire hazard. Another problem with solvent-based systems is that it is very difficult, if not impossible, to remove all of the solvent from the applied chemical treatment. Therefore, solvent-based chemical treatments are limited, as a practical matter, to those systems where any residual solvent left behind in the coating of organic material remaining on the fibers will not have a significantly adverse affect.
With prior melt-based chemical treatments, thermoplastic-type organic solids are melted and applied to the glass fibers. U.S. Pat. Nos. 4,567,102, 4,537,610, 3,783,001 and 3,473,950 disclose examples of such chemical treatments. One disadvantage of prior melt-based processes is the energy costs associated with melting the chemical treatments. The organic solids used with prior melt-based systems are melted at relatively high temperatures in order for the melted organic solids to be applied to the glass fibers. The high temperatures are needed because the organic solids used in the past have relatively high molecular weights. Such high melt temperatures also pose the risk to workers of being burned by the equipment used to melt the plastic material and by the molten plastic material itself. In addition, specialized equipment is typically needed to apply and otherwise handle the high-temperature molten plastic material.
The radiation cure-based chemical treatments are typically acrylate-based organic chemicals, either with or without a solvent, which are cured with ultraviolet radiation via a photoinitiator. U.S. Pat. Nos. 5,171,634 and 5,011,523 disclose examples of such chemical treatments. A major disadvantage to processes using such chemical treatments is that the radiation used, such as ultraviolet radiation, and the chemical treatment used, such as acrylates, are relatively hazardous, often requiring special handling and safety precautions. Some of these processes, such as that disclosed in U.S. Pat. No. 5,171,634, require the radiation curing to be repeated a number of times to obtain the maximum benefit. Each additional radiation-curing step increases the risks involved and adds additional cost to the process. Furthermore, radiation-curable thermoset plastics, and their requisite photoinitiators, represent a highly specialized area of thermoset chemistry. As a consequence, such radiation-cured chemical treatments are expensive and not generally compatible with various classes of matrix resins.
In order to fabricate composite parts, the strands of glass fibers are often further chemically treated in an off-line impregnation process with a polymeric resin. The resin can be a thermoset, either one- or two-part, or a thermoplastic. In one example, previously formed and sized continuous glass fibers are impregnated with a thermosetting resin and then pulled through a heated pultrusion die to cure the resin and make the composite article, such as ladder rails. In such an off-line process, the continuous glass fibers must be separated in some manner to allow impregnation of the resin between the fibers and then recombined. This requirement almost always results in the use of additional hardware such as spreader bars, impregnation baths, and drying or curing ovens. These types of processes have the disadvantage that they add cost and complexity to the process. In addition, the resultant extra handling of the glass fibers can cause breakage of the individual glass filaments, and thereby a degradation in the properties of the composite article. Therefore, while such off-line processes may be effective, they are time-consuming and inefficient (e.g., requiring additional process steps) and, thus, expensive.
Accordingly, there is a need in the art for a safer, more efficient and more cost-effective process for applying a chemical treatment to glass fibers, where the viscosity of the chemical treatment is low enough to sufficiently wet the glass fibers without the need for a solvent, where the chemical treatment does not require radiation curing and the viscosity of the applied chemical treatment increases with very little, if any, generation of water, volatile organic carbon (VOC) or other solvent vapor, and where the resulting chemically treated glass fibers are suitable for subsequent processing into a composite article. There is also a need for an in-line process for forming a preimpregnated glass composite strand from a plurality of continuously formed glass fibers which are chemically treated in this manner, where the resulting prepreg strand is suitable for subsequent in-line or off-line processing into a composite article.
The use of composites having fiber-reinforced polymeric matrices is widespread. Fiber-reinforced polymeric composite products have been manufactured using a variety of processes and materials. As referred to above, one such process involves impregnating one or more strands or bundles of reinforcing fibers (e.g., glass fibers, synthetic fibers or some other reinforcing fibers) with a thermoplastic material, and using the resulting composite strands to mold a composite article. These composite strands have been used in the form of continuous threads (i.e., long lengths of strand) and discrete pellets (i.e., short lengths of strand). The fibers from the composite strands provide the reinforcement and the thermoplastic material forms at least part of the matrix for the composite article.
It is desirable for each fiber strand to be fully impregnated with the thermoplastic matrix material, that is, for the thermoplastic material essentially to be evenly distributed throughout each bundle of fibers and between the fibers. Because all of the fibers start out surrounded by matrix material, the fully impregnated fiber strands can be molded less expensively and more efficiently and the corresponding composite article can exhibit improved properties. However, it is difficult and time-consuming to fully impregnate fiber strands with typical thermoplastic matrix materials (e.g., engineering thermoplastics). Fully impregnating strands at high throughput rates has been particularly difficult, especially at the throughput rates typically experienced during the production of continuously formed glass reinforcing fibers.
In an effort to fully impregnate continuously formed glass fiber strands, the number of fibers used to form each strand (i.e., fiber density) has been reduced from a typical density of about 2000 fibers/strand to 1200 fibers/strand or less, to reduce the time it takes to impregnate each fiber strand. However, by reducing the number of fibers in each strand being processed at a given time, the production output and cost efficiency of the process can be adversely impacted. In addition, fully impregnating even such lower density strands is still sufficiently time-consuming to prevent even the lower density strands from being fully impregnated and processed at the higher throughput rates typically approached in the production of continuous glass reinforcing fibers.
In an effort to obtain higher throughputs, one prior process only partially impregnates the fiber strand and coats the strand in a uniform layer of thermoplastic matrix material, leaving a central core of fibers not impregnated with the thermoplastic. This coating and partial impregnation of the strand is accomplished by pulling the strand through what has been referred to as a "wire-coating" device. Wire-coating devices, such as that disclosed in U.S. Pat. No. 5,451,355, typically include an extruder for supplying molten thermoplastic matrix material and a die having an entrance orifice, an exit orifice and a coating chamber disposed therebetween. The extruder supplies molten thermoplastic material to the coating chamber. The strand is coated and partially impregnated with the thermoplastic matrix material as it passes through the coating chamber, and the coating is formed into a uniform layer when the coated strand passes through the exit orifice of the die. The resulting coated strand is either used in the form of a thread (e.g., in compression-molding applications) or cut into discrete pellets (e.g., in injection-molding applications). Because the strand is only partially impregnated with the thermoplastic matrix material, the strand can be processed at relatively high throughputs.
However, these partially impregnated wire-coated strands also exhibit a number of problems because of their central core of unimpregnated fibers. When in pellet form, the fibers in the central unimpregnated core tend to fall out of the thermoplastic coating. When the strand is in the form of a thread, the core fibers are less likely to fall out, but the core of these wire-coated threads must still be impregnated at some point to optimize the properties of the resulting composite article. Impregnating the central core of such wire-coated threads during the molding operation can be difficult and time-consuming, if not impossible as a practical matter. Thus, molding with such wire-coated threads can cause a reduction in the overall production rates, rather than an increase as desired.
Therefore, there is a need for a way to produce fully impregnated fiber strands at higher throughput rates, even when each strand has a relatively high fiber density, where the resulting composite strands, either in thread or pellet form, are suitable for molding fiber-reinforced thermoplastic articles.